1. Field of Invention
The present invention relates to a transmission, particularly a tension element drive system for a robotic arm.
2. Description of Related Art
Robotic systems are often used in applications that require a high degree of accuracy and/or precision, such as surgical procedures or other complex tasks. Such systems may include various types of robots, such as autonomous, teleoperated, semi-active, passive, and interactive. For example, in joint replacement surgery, a surgeon can use an interactive, haptically guided robotic arm in a passive manner to sculpt bone to receive a joint implant, such as a knee implant. To sculpt bone, the surgeon manually grasps and manipulates the robotic arm to move a cutting tool (such as a burr) that is coupled to the robotic arm. As long as the surgeon maintains the cutting tool within a predefined virtual cutting boundary, the robotic arm moves freely with low friction and low inertia such that the surgeon perceives the robotic arm as weightless and can move the robotic arm as desired. If the surgeon attempts to cut outside the virtual cutting boundary, however, the robotic arm provides haptic (or force) feedback that prevents or inhibits the surgeon from moving the cutting tool beyond the virtual cutting boundary. In this manner, the robotic arm enables highly accurate, repeatable bone cuts.
The ability of a robotic arm to function in the above-described manner is dependent on the drive system (also called the drive train or drive transmission) of the robotic arm. Ideally, the drive system is characterized by low friction, low inertia, high stiffness, large bandwidth, near-zero backlash, force fidelity, and/or backdrivability. A flexible transmission, such as a tension element drive system, may have these characteristics. One difficulty with conventional tension element drive systems, however, is that they may not be sufficiently fail-safe for use in surgical applications where failure of the drive system could endanger a patient. For example, failure of one tension element (e.g., a cable or cord) in the drive system could result in unintended movement of the robotic arm that could harm the patient. To improve safety, the robotic arm can include joint brakes to constrain movement of the joints of the robotic arm in the event of a tension element failure. Incorporation of joint brakes, however, increases the weight and inertia of the robotic arm, which adversely impacts backdrivability and haptic response.
Another difficulty with conventional tension element drive systems is that the tension elements must be pre-tensioned to eliminate slack that may cause backlash. Pre-tensioning loads, however, are about 15% to 50% of the breaking strength of the tension element, which imparts large forces to drive system components, bearings, and support structure. The high load also increases friction forces in the drive system components and contributes to surgeon fatigue.
Another difficulty with conventional tension element drive systems is that such drive systems may not be easily manufactured, serviced, and/or upgraded. For example, a conventional tension element drive system may be an integral system in the sense that components in one part of the drive system (e.g., in one joint) are, to some degree, dependent on or impacted by components in another part of the drive system (e.g., in another joint). Thus, if one portion of the drive system is defective, it may be necessary to dismantle other portions of the drive system that are functioning properly in order to repair the defective portion. For example, repairing a problem in one joint of the robotic arm may require de-cabling multiple joints of the robotic arm. The inability to isolate portions of a conventional tension element drive system increases the time and labor required to service and upgrade the robotic arm, which results in costly repairs and lengthy downtime that reduces a hospital's ability to optimize use of the robotic arm.